1. Field of the Invention
The present invention relates to a pressure sensor for converting a fluid pressure to an electric signal for measurement and a manufacturing method of the pressure sensor.
2. Description of Related Art
Traditionally, there has been used a pressure sensor for measuring pressure of a fluid such as gas and liquid, in which the fluid pressure is converted to an electric signal for measurement.
For instance, there is known a pressure sensor disclosed in Document 1: JP-A-2002-195902.
The pressure sensor includes: a metal stem having a diaphragm; a strain gauge bonded on the diaphragm; and a housing for introducing fluid into the metal stem, which is adapted for measuring the pressure of the fluid flowing in a pipe.
An opening of the metal stem is connected to a pressure-introducing channel of the housing by metal sealing, where the fluid is introduced inside the diaphragm of the metal stem via the pressure-introducing channel of the housing.
When the diaphragm is deformed on account of the pressure of the fluid introduced inside the metal stem, the strain gauge detects the strain as an electric signal. Then, the electric signal is amplified and converted to obtain a pressure value of the fluid.
Document 2 (JP-A-2006-38538) discloses a pressure sensor including: a bottomed cylindrical member having a thin bottom; a pressure detector provided at the bottom of the bottomed cylindrical member; and a pressure-introducing joint for introducing a fluid into the bottomed cylindrical member, the bottomed cylindrical member and the pressure-introducing joint being butt-welded.
According to the above arrangement, since the bottomed cylindrical member of a material with excellent mechanical strength and corrosion resistance and the pressure-introducing joint made of a material that is expensive and still is superior in corrosion resistance are welded, a pressure sensor with excellent corrosion resistance can be provided with low cost.
Incidentally, the traditional pressure sensor as disclosed in the Document 1 requires that the metal stem and the housing are rigidly fixed with a screw in order to prevent leakage of the fluid, so that contact surfaces between the metal stem and the housing are sealed in a collapsed manner.
Here, in order to ensure the sealing, the metal stem and the housing have to be processed with high accuracy. Further, extremely large force has to be applied with utmost attention in assembling the components.
On the other hand, when the metal stem and the housing are fastened with strong force, the metal stem is strained, which is transferred to the strain gauge to cause great deviation on the output thereof. Accordingly, a processing circuit for an electric signal has to be provided in order to correct the output deviation.
On the other hand, when different materials are welded as in the pressure sensor disclosed in the Document 2, strain is generated on account of the difference between the coefficients of thermal expansion of the materials. Such welding strain is smaller than the strain on account of fastening, however, is not small enough to be ignored in measurement. Accordingly, a correction process by the electric signal processing circuit is eventually required.
In addition, it is considered that such welding strain is gradually relaxed in accordance with aging, so that long-term stability of measurement may be impaired.
Incidentally, even when the same kind of materials are to be welded, strain occurs on account of, for example, evaporation of a part of molten metal during welding to cause volume reduction of the welded portion.
As an alternative option for preventing output deviation on account of strain of the component, it is possible that the bottomed cylindrical member is sufficiently lengthened to prevent transfer of the strain of the cylindrical portion of the bottomed cylindrical member to the strain gauge. However, such option is far from practical in view of increase in the entire size of the pressure sensor.